The invention relates to a method and to an apparatus for welding a joint, particularly for joining thermoplastics, with a beam of energy whose wavelength may be in or near the infrared range. The short-wave radiation may be in the range of 0.7 to 2.0 xcexcm. The two joint zones or joint faces are held coincidental or in contact with each other during plastification. In this or some other way first one joint zone is plasticized before then the other joint zone is plasticized. Thereby the second joint zone is wetted with the plasticized first joint zone and is thus likewise plasticized by heat conduction.
In butt welds both joint zones are plasticized separate from each other, but simultaneously, and then pressed against each other in the plastic state to become welded. Due to the contact pressure plasticized material may break out from the joint and then form a bead which is usually undesirable. The heating effect may also be done by vibration, namely by friction at the joint face or by friction of both joint faces. This results in abrasion particles which interfere and are difficult to eliminate from many products. In penetration beam welding, by contrast, the welding energy is applied to the coincident joint faces, for example through the exposed cross-section and its joint face onto the joint zone of the covered cross-section. As a result of this the joint zone adjoining the joint face is plasticized as a partial or flux layer of the cross-section. Thereafter the plasticized material mass moistens by flux, mutual transverse motion or contact pressure the other joint face and plasticizes also the joint zone thereof, resulting in the weld. When this weld is made along the joint or seam progressively, then field sections of the seam are already solidly welded whilst others are still plastic or in need of being plasticized. This makes homogenous contact pressure of the joint faces difficult in the various sections of the seam like also levelling of the joint faces by melting down. Melting down enables to equalize differences in tolerance of the shape and location of each joint face.
It is conceivable to beam the complete joint zone simultaneously with energy, for example by an array of diode lasers or beam focal points. This needs a complicated apparatus especially when the run of the weld is not straight. Apart from this the beam output is not suitable for workpieces differing in shape.
An object of the invention is to provide a method respectively an apparatus with which the disadvantages of known configurations or the aforementioned kind can be obviated. Another object is to make it possible to soften or maintain plasticized each of the two joint faces over the full seam length substantially simultaneously. Thus in this state the two cross-sections or components to be joined should still be mutually positionable into that predetermined orientation in which they are to be finally joined by the solid weld. A further object is to achieve a seam which remains sealed and has tensile strength even when exposed to high loads or pressures. Namely the same strength as of parts of the cross-sections adjoining the seam or spacedly juxtaposed to the seam is intended. Still another object is to make the joint with little consumption, irrespective of the seam course.
According to the invention only one or both of the joint faces is/are entirely preheated to an intermediate temperature slightly below the working or melting temperature. At the latest then the joint faces are brought into mutual contact and the entire preheated joint face is heated to the final melting temperature. The melt or melting mass then heats the other joint face simultaneously over the entire seam length and up to the melting temperature. This results in the melt mass of the two joint zones intermingling homogeneously and solidifying on cooling to a solid weld. This prevents the melts from solidifying in some seam area before both joint zones of all other seam areas have attained the melting temperature, namely are fused together. All seam areas consequent melt near to simultaneously or each seam area fuses as long as all remaining seam areas are still soft enough to allow the components to be mutually moved so that the softened material is able to yield or be displaced. The seam melt solidifies over the full length and width of the seam simultaneously or practically at the same time thus also avoiding strains to rise.
Preheating may be done in several steps or cumulative from a first intermediate temperature to a next higher intermediate temperature. For this purpose the beam focus or field is moved along the joint zone so quickly that each section of the joint zone between two sequential heatings by the beam field cannot cool down to the last initial or minimum temperature of the previous heating. For example, the heatings may be done in one, two or three seconds once, ten or more times, whereby also every whole number between one and twelve is possible. This incremental or gradual heating of the joint zone occurs whilst the joint faces are mutually urged in contact. As soon as the one joint face has become soft or plasticized simultaneously over the full length of the seam, the associated component can be moved by the contact pressure over a desired degree relative to the other component. This enables tolerance differences in the dimensions of the components or in the superficial shape of the joint faces to be equalized. This motion is limited either by stops on the components themselves or by stops on the pressing/gripping device with which fixture the components are held or pressed together during production of the joint.
During preheating and/or melting the energy beam penetrates the one joint face, which it does not heat or only slightly so due to its physical properties. The beam impinges the joint face directly juxtaposed which due to its physical properties absorbs the radiation in heating up until it enters into the melting phase which heats the first joint face likewise up to plastification. Thus even long welds can be produced in less than 15, 10 or 5 seconds including preheating. The method is particularly suitable for plastics or for circumferentially continous seams as used in joining container components. It is also suitable for other purposes, e.g. for complementary preadaptation of joint faces where these need to be non-distructively separable.
The method is implementable with known apparatuses, namely with a writing or scanner head having a beam output for an energy beam such that the beam can also be deflected continuously in differing directions with the head stationary. However, the beam output may also be secured to a robotic arm to be thereby moved articulatedly and powered in all three dimensional directions so that the beam is moved along the seam by the beam output which is held at a constant spacing from the seam. According to the invention the apparatus has control means which guide the beam field multiply in sequence over each of the sections of the joint zone or seam, particularly in timed intervals of maximally one to six or four seconds. The beam output may be formed by a mirror or a focussing lens. Where for beam steering two sequential mirrors are used, each is swivable about a separate axis independently of the other. Thus the beam can be moved simultaneously in two spatial axes which are mutually perpendicular. With the focussing optics or a Z-axis module the beam field can also be powered in the third spatial axis whilst maintaining its field area constant. Thus varying distances between beam output and joint zone are compensated.